An antenna plays a critical role for the transmission and receipt of electromagnetic energy in a wireless communication system. The electric characteristics of the antenna have a significant influence on the quality of communication, and are an indication for quality of wireless signal receipt and transmission. In various applications of products for transmission/receipt of wireless signals, antennas of various materials and configurations have been used. Proper selection of antennas can enhance the overall outside appearance of an electronic product that incorporates the antenna and also improve transmission of wireless signals, as well as reduce overall costs of the whole wireless facility.
Besides being good in wireless transmission and receipt, matching with the electronic product in which an antenna is included is also an important issue for the antenna. For example, for a mobile phone of which the appealing factors are being compact and light weight, and other portable wireless electronic device, such as a notebook computer, the use of an antenna must take into consideration both the overall outside appearance of the electronic product and excellent performance of signal transmission and receipt. Manufacturers of electronic products of these kinds have put in substantial effort to make the antennas of these products minimum and compact.
To make an antenna compact and minimized, a planar inverted-F antenna (PIFA) has been proposed. The PIFA has a nearly omni-directional radiation field and simple construction and has an operation length of around a quarter of the operation wavelength. Thus, the PIFA is most fit for Bluetooth devices, mobile phones, and other portable wireless electronic devices. Further, a PIFA can be made by simply using a metal conductor to which feeding element is provided and which is connected to the ground via short-circuit elements. Thus, the manufacturing cost is extremely low. In addition, the PIFA can be directly bonded by soldering to a circuit board of the electronic product.
A conventional PIFA comprises a ground plane, a short circuit piece, and a planar radiating plate, wherein the planar radiating plate is provided, at a predetermined location, with and connected to a signal transmission line. Such a predetermined location serves as a feeding point of the PIFA.